Corrosion inhibitor of NiCu for high performance writers

ABSTRACT

The problem of copper corrosion that occurs in the presence of strong alkaline developing solutions during photo rework has been overcome by protecting all exposed copper bearing surfaces from attack. Two ways of achieving this are described. In the first method, benzotriazole (BTA) is added to the developing solution which is then used in the normal way, developing time being unaffected by this modification. In the second method, the surface that is to receive the photoresist is first given a dip in a solution of BTA, following which the photoresist is immediately applied and processing, including development, proceeds as normal. For both methods the result is the elimination of all copper corrosion during development.

FIELD OF THE INVENTION

[0001] The invention relates to the general field of photoresistprocessing in the presence of copper, with particular application to themanufacture of write heads for magnetic disk systems.

BACKGROUND OF THE INVENTION

[0002] Referring to FIG. 1, we show, in schematic representation, across-sectional view of a write head for a magnetic disk system. Themagnetic field needed to perform the write operation is generated byflat coil 16 made up of a number of turns, with 13 being an example ofone side of a single turn. Surrounding the flat coil is magneticmaterial comprising upper and lower pole pieces 12 and 11 respectively.These pole pieces are joined at one end (on the left in this figure) andare separated by small gap 14 at the other end. The magnetic field thatis generated by flat coil 16 ends up being concentrated at gap 14. It issufficiently powerful that the fringing field that extends outwards awayfrom gap 14 is capable of magnetizing the magnetic storage medium overwhose surface 15 the head ‘flies’. The distance between gap 14 andsurface 15 is typically between about 10 and 50 nm.

[0003] In the course of manufacturing the various layers that make upthe gap region it is often found to be convenient to use copper, eitheralone or in combination with other materials, in one or more of thelayers. As is well known, most or all of these layers will be shaped bymeans of photolithography. Additionally, a given photolithographic stepmay not always be implemented exactly as intended, for examplemisalignment between related structures may have occurred. Under suchcircumstances it is often possible to strip the unsatisfactory layer ofphotoresist and repeat the photolithographic step, a process referred toas photo rework.

[0004] Certain types of photoresist (notably positive resists) aredeveloped using solutions that are very alkaline, typically having a pHin the range from 10 to about 14. In most cases this poses no problems.However, when one of the afore-mentioned pure or partial copper layerscomes into contact with such a high pH solution, it is subject toattack. Thus, when using alkaline developers, any exposed copper musteither be kept away from the developer or some way must be found torender the copper immune to attack. The present invention discloses asolution that is based on the second of these alternatives.

[0005] No references that describe the exact process of the presentinvention were uncovered in the course of a routine search of the priorart. Several references of interest were, however, found. For example,U.S. Pat. No. 5,236,552 (Fang) shows a photoresist stripping solutioncontaining a corrosion inhibitor such as BTA. U.S. Pat. No. 5,863,710(Wakiya et al.) shows a developer solution with an aluminum corrosioninhibitor. U.S. Pat. No. 5,635,339 (Murray) shows a photo-thermographicelement using BTA as a co-developer. U.S. Pat. No. 5,316,573 (Brusic etal.) forms a corrosion inhibiting layer by dipping in a solution ofcopper and BTA. U.S. Pat. No. 5,304,252 (Condra et al.) shows a maskremoving process with a corrosion inhibitor of BTA for a printed circuitboard.

SUMMARY OF THE INVENTION

[0006] It has been an object of the present invention to provide aprocess for developing photoresist in the presence of copper bearingmaterial without corroding the latter.

[0007] Another object of the invention has been to be able to performphoto rework in the gap region of the write head of a magnetic disksystem without affecting copper bearing surfaces nearby.

[0008] These objects have been achieved by protecting all exposed copperbearing surfaces from attack by strongly alkaline solutions of the typeused to develop photoresist. Two ways of achieving this are described.In the first method, benzotriazole (BTA) is added to the developingsolution which is then used in the normal way, developing time beingunaffected by this modification. In the second method, the surface thatis to receive the photoresist is first given a dip in a solution of BTA,following which the photoresist is immediately applied and processing,including development, proceeds as normal. For both methods the resultis the elimination of all copper corrosion during development.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a schematic cross-section of a write head for a magneticdisk system.

[0010]FIG. 2 is a closeup view of the gap region of the write head seenin FIG. 1.

[0011]FIG. 3 shows the object illustrated in FIG. 2 viewed from adifferent direction.

[0012]FIG. 4 shows a photoresist frame used for forming a non-magneticstep layer.

[0013]FIG. 5 shows the molecular structure of BTA.

[0014]FIG. 6 is a polarization curve for a nickel-copper alloy indeveloping solutions with and without added BTA.

[0015]FIG. 7 plots the exchange current as a function of copper contentfor a nickel-copper alloy in developing solutions with and without addedBTA.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] In FIG. 2 we show (schematically) a closeup view of the gapregion that is part of FIG. 1. In order to concentrate the magnetic fluxto the maximum extent possible, the size of the upper pole piece isreduced in the immediate vicinity of the gap. Thus, in addition to theupper and lower pole pieces, 12 and 11 respectively, seen in FIG. 1, asecond upper pole piece 21 has been introduced between pole piece 12 andthe gap layer 23, the latter being a layer of an insulating materialsuch as alumina or nickel-copper. As can be seen, second pole piece 21has been further reduced in cross-sectional area where its lower surfacecontacts gap layer 23.

[0017] This reduction in area of the lower portion of 21 is convenientlyimplemented by introducing a layer of non-magnetic material, in the formof step 22, prior to the formation of 21. Because it can be processed ina manner similar to nickel-iron, nickel-copper (between about 40 and 70weight % copper) has been found to be a suitable material from which tobuild the non-magnetic step 22.

[0018]FIG. 3 is a cross-section of FIG. 2 made through plane 3-3. Oncelower pole piece 11 is in place, a layer 23 of gap material (such asalumina or nickel copper) is laid down to a thickness between about1,000 and 4,000 Angstroms.

[0019] Referring now to FIG. 4, in order to form non-magnetic step layer22, layer 41 of photoresist is first laid down and then patterned toform frame 43 inside which layer 22 can now be grown. With layer 22 inplace, the same frame can be used to contain layer 21 while it is beinggrown. However, it is possible that the exact position of frame 43relative to other parts of the structure is unsatisfactory, in whichcase photo rework becomes necessary.

[0020] It then becomes necessary to remove photoresist layer 41 andreplace it with a fresh second photoresist layer which can be re-exposedand developed so as to locate frame opening 43 where it belongs. Aspreviously noted, the developer is strongly alkaline so some of thecopper within layer 22 is liable to be attacked, resulting inundesirable corrosion of the top surface of step 22. The presentinvention provides two ways to protect the copper from attack by thedeveloper which we now disclose as two separate embodiments.

First Embodiment

[0021] In this approach, we modify the developer itself by adding to ita complexing agent that protects the copper from the alkaline solution.Although any one of several possible complexing agents could have beenused, our preferred choice has been benzotriazole (BTA) which is addedto a standard water based developer (such as LDD-26W) at a concentrationbetween about 0.2 and 2 gms./liter. The molecular structure of BTA isillustrated in FIG. 5. The modified developing solution for thephotoresist is then used in the normal way. That is, the photoresist isdeveloped for a time period that is between about 2 and 10 minutes, thisbeing the same time as that required for development in a developingsolution having no BTA. Thus, the process is essentially unchanged,except that the copper corrosion no longer occurs.

Second Embodiment

[0022] In the second embodiment, the photoresist developer is notmodified. Instead, after removing the misaligned layer of photoresist,the structure is dipped in an aqueous solution of BTA (at aconcentration of between about 0.5 and 2 gms./liter for between about0.5 and 15 minutes. It is then immediately coated with a fresh layer ofphotoresist which is then exposed and developed in the usual way. Thepresence of the BTA solution at the interface between the photoresistand the copper-bearing layer is then sufficient to prevent any corrosionof the copper from occurring during photoresist development. Note thatthe photoresist is developed for a time period that is between about 2and 10 minutes, this being the same time as that required fordevelopment even if no pre-dip in a BTA solution has taken place.

[0023] For both embodiments, following the successful development of thephotoresist, the new, correctly located, frame is now in position sothat part 12 of the upper pole piece can be formed without part 21 beingattacked. Finally, the reworked layer of photoresist is removed.

[0024] By way of explaining why the process of the present invention iseffective in suppressing the dissolution of copper in alkaline solution,we refer to FIG. 6. which is a polarization curve for Cu₆₀Ni₄₀ in anunmodified developing solution (curve 61) and Cu₆₀Ni₄₀ in a developingsolution to which 1.0 gm/liter of BTA has been added (curve 62). In bothcases a plot was made of Log (I/A), where I and A are current and arearespectively, vs voltage between an electrode made of the NiCu alloy anda standard calomel reference electrode. These curves show that theexchange current decreases by 1½orders of magnitude when BTA is added.This shows that the addition of the BTA has substantially reduced thedissolution rate of the NiCu.

[0025] In FIG. 7 we plot the exchange current density (log I/A) as afunction of copper content within the NICu alloy. Curve 71 is for theunmodified developer while curve 72 is for the developer with 1 gm/literof BTA added. This data shows that the exchange current will decreasewith BTA addition for NiCu films having different copper concentrations.This demonstrates that BTA addition is effective to inhibit corrosion ofNiCu films over the full range of copper content.

[0026] While the invention has been particularly shown and describedwith reference to the preferred embodiments thereof, it will beunderstood by those skilled in the art that various changes in form anddetails may be made without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A process for developing photoresist in the presence of copper, comprising: providing a surface that includes some copper; providing a developing solution for the photoresist; modifying said developing solution by adding to it a quantity of BTA; coating said surface with the photoresist and then exposing the photoresist; and using the modified solution, developing the photoresist whereby any of the copper that comes into contact with the modified developing solution is unaffected by the development process.
 2. The process of claim 1 wherein the quantity of BTA that is added makes its concentration in the developer to be between about 0.2 and 2 gms./liter.
 3. The process of claim 1 wherein the developing solution has a pH between about 10 and
 14. 4. The process of claim 1 wherein the resist is a positive photoresist.
 5. The process of claim 1 wherein the photoresist is developed for a time period that is between about 2 and 10 minutes, said time period being the same as that required for development in a developing solution having no BTA.
 6. A process for pre-wetting a surface that contains some copper, comprising: dipping said surface into an aqueous solution of BTA; then coating said surface with a layer of photoresist; and exposing the photoresist and then developing it whereby, when development has been completed, the copper is unaffected.
 7. The process of claim 6 wherein the BTA has a concentration in the aqueous solution of between about 0.5 and 2 gms./liter.
 8. The process of claim 6 wherein the pH of the developing solution is between about 10 and
 14. 9. The process of claim 6 wherein the resist is a positive photoresist.
 10. The process of claim 6 wherein the surface is dipped in the solution of BTA for between about 0.5 and 15 minutes.
 11. The process of claim 6 wherein the photoresist is developed for a time period that is between about 2 and 10 minutes, said time period being the same as that required for development in a developing solution having no BTA.
 12. A process for doing photo re-work during manufacture of a write-head for a magnetic disk system, comprising: providing a lower pole piece; depositing a layer of gap material on the lower pole piece; depositing and patterning a first layer of photoresist on the layer of gap material, thereby forming a first frame; within said first frame, forming a step layer of a non-magnetic material that includes copper; removing said first layer of photoresist; depositing and exposing a second layer of photoresist on the step layer; providing a developing solution for the photoresist; modifying said developing solution by adding to it a quantity of BTA; using the modified solution, developing the second layer of photoresist, whereby any copper in the step layer that comes into contact with the modified developing solution is unaffected by the development process, thereby forming a second frame. within said second frame, forming an upper pole piece layer; and removing said second layer of photoresist.
 13. The process of claim 12 wherein the quantity of BTA that is added makes its concentration in the developer to be between about 0.2 and 2 gms./liter.
 14. The process of claim 12 wherein the resist is a positive photoresist.
 15. The process of claim 12 wherein said non-magnetic material is an alloy of copper and nickel containing between about 40 and 70% copper.
 16. A process for doing photo re-work during manufacture of a structure that is part of a write-head for a magnetic disk system, comprising: providing a lower pole piece; depositing a layer of gap material on the lower pole piece; depositing and patterning a first layer of photoresist on the layer of gap material, thereby forming a first frame; within said first frame, forming a step layer of a non-magnetic material that includes copper; removing said first layer of photoresist; dipping the structure in an aqueous solution of BTA; then coating the structure with a second layer of photoresist; exposing the second layer of photoresist and then developing it, whereby any copper in the step layer that comes into contact with developing solution is unaffected by the development process, thereby forming a second frame. within said second frame, forming an upper pole piece layer; and removing said second layer of photoresist.
 17. The process of claim 16 wherein the BTA has a concentration in the aqueous solution of between about 0.5 and 2 gms./liter.
 18. The process of claim 16 wherein the resist is a positive photoresist.
 19. The process of claim 16 wherein the surface is dipped in the solution of BTA for between about 0.5 and 15 minues.
 20. The process of claim 16 wherein said non-magnetic material is an alloy of copper and nickel containing between about 40 and 70% copper. 